1. Field of the Invention
The present invention relates to a radio receiver, or more particularly to a high frequency amplifier circuit of an electronically-tuned, car-mounted AM radio with improved characteristics at a large input.
2. Description of the Prior Art
In an electronically-tuned radio receiver a variable capacity diode is used as the channel selection element. The diode produces nonlinear distortions, because the capacity varies with the voltage applied. Since the antenna is capacitive in a car-mounted radio, the diodes and the antenna cannot be directly combined to compose a tuning circuit. Accordingly, an input circuit as shown in FIGS. 1 or 2 is used. In FIG. 1 an antenna is connected to a terminal 10, the antenna output is amplified by a field effect transistor 12, and a LC resonance circuit 14 is used for turning. A high frequency amplifier 16 is connected to the LC resonance circuit, and the output from the high frequency amplifier 16 is provided to a resonance circuit 18. The output of the circuit 18 is introduced into a mixer MIX in the next stage. AGC voltage is applied to the amplifier 16, and also the transistor 12 by way of a resistance 20. Since this circuit is of the untuned input type and unncessary waves are fed into the transistor 12 and amplified together with desired waves, the characteristics at large inputs, especially cross modulation characteristics, are inferior.
In FIG. 2, a resonance circuit 22 and a negative feedback resistance 30 are connected to the source of the transistor 12, in which when desired waves are applied, the circuit 22 resonates and the impedance becomes zero so that negative feedback is not applied, and when unnecessary waves are fed, a negative feedback is applied so that the wave will not be amplified as much. As a result, the performance is improved with respect to disturbance waves (unnecessary waves), but the following problems occur. Desired waves are free from negative feedback and are amplified by the transistor 12. Since gm of the transistor 12 is nonlinear and finite, and the Q and gain of resonance circuits 14, 22 depend on gm, the cross modulation characteristics are not sufficient. In particular, when the resonance circuit 14 resonates at desired waves, the resonance circuit 22 falls to zero impedance and receives a high voltage, so that the varactor diode used as its capacitive element builds up a large nonlinear distortion. Yet in this circuit, since it is difficult to apply negative feedback to the input side of transistor 12, the transistor 24 is connected to the output side as an attenuator. In this arrangement, however, the transistor 24 lowers the Q of resonance circuit 14, and its own nonlinear form is introduced into the circuit.